Uncontrolled hypertension (ie, a 24-hour ambulatory systolic blood pressure of ≥130 mm Hg and diastolic blood pressure of ≥80 mm Hg or clinic systolic blood pressure of ≥140 mm Hg and diastolic blood pressure of ≥90 mm Hg) in young adults is a US public health burden. To evaluate the effect of a telephone coaching and blood pressure self-monitoring intervention compared with usual care on changes in systolic and diastolic blood pressures and behaviors at 6 and 12 months. This randomized clinical trial included male and female participants aged 18 to 39 years with uncontrolled hypertension confirmed by 24-hour ambulatory blood pressure testing. This was a geographically diverse, multicentered study within 2 large, Midwestern health care systems. Data were collected from October 2017 to February 2022 and analyzed from February to June 2022. The My Hypertension Education and Reaching Target (MyHEART) intervention consisted of telephone coaching every 2 weeks for 6 months, with home blood pressure monitoring. Control participants received routine hypertension care. The co-primary clinical outcomes were changes in 24-hour ambulatory and clinic systolic and diastolic blood pressure at 6 and 12 months. The secondary outcomes were hypertension control (defined as ambulatory systolic blood pressure <130 mm Hg and diastolic blood pressure <80 mm Hg or clinic systolic blood pressure <140 mm Hg and diastolic blood pressure <90 mm Hg) and changes in hypertension self-management behavior. A total of 316 participants were randomized (159 to the control group and 157 to the intervention group) from October 2017 to December 2020. The median (IQR) age was 35 (31-37) years, 145 of 311 participants (46.6%) were female, and 166 (53.4%) were male; 72 (22.8%) were Black, and 222 (70.3%) were White. There were no differences in baseline characteristics between groups. There was no significant difference between control and intervention groups for mean 24-hour ambulatory systolic or diastolic blood pressure or clinic systolic or diastolic blood pressure at 6 or 12 months. However, there was appreciable clinical reduction in blood pressures in both study groups (eg, mean [SD] change in systolic blood pressure in intervention group at 6 months, -4.19 [9.77] mm Hg; P < .001). Hypertension control did not differ between study groups. Participants in the intervention group demonstrated a significant increase in home blood pressure monitoring at 6 and 12 months (eg, 13 of 152 participants [8.6%] checked blood pressure at home at least once a week at baseline vs 30 of 86 [34.9%] at 12 months; P < .001). There was a significant increase in physical activity, defined as active by the Godin-Shephard Leisure-Time Physical Activity Questionnaire, in the intervention group at 6 months (69 of 100 [69.0%] vs 51 of 104 [49.0%]; P = .004) but not at 12 months (49 of 86 [57.0%] vs 49 of 90 [54.4%]; P = .76). There was a significant reduction in mean (SD) sodium intake among intervention participants at 6 months (3968.20 [1725.17] mg vs 3354.72 [1365.75] mg; P = .003) but not 12 months. There were no significant differences in other dietary measures. The MyHEART intervention did not demonstrate a significant change in systolic or diastolic blood pressures at 6 or 12 months between study groups; however, both study groups had an appreciable reduction in blood pressure. Intervention participants had a significant reduction in dietary sodium intake, increased physical activity, and increased home blood pressure monitoring compared with control participants. These findings suggest that the MyHEART intervention could support behavioral changes in young adults with uncontrolled hypertension. ClinicalTrials.gov Identifier: NCT03158051.